Rainfall conditions, runoff, and sediment loss
Eight rainfall events were observed in the experiment (Table 2). Four of these events generated runoff, two of which were continuous rainfall pattern, while the other two were intermittent rainfall pattern. Intermittent rainfall represents multiple rainfall events in a day, whereas continuous rainfall represents only one rainfall event in a day.
Table 2 Rainfall conditions.
Date
|
Precipitation
(mm)
|
Average rainfall intensity (mm·min-1)
|
Rainfall pattern
|
2016–07–21
|
67.0
|
0.08
|
Intermittent rainfall
|
2016–07–22
|
28.0
|
0.04
|
Continuous rainfall
|
2016–07–25
|
49.2
|
0.04
|
Intermittent rainfall
|
2016–07–26
|
25.2
|
0.03
|
No runoff initiation
|
2016–08–01
|
5.2
|
0.02
|
No runoff initiation
|
2016–08–02
|
0.2
|
0.10
|
No runoff initiation
|
2016–08–07
|
16.4
|
0.05
|
No runoff initiation
|
2016–08–13
|
24.6
|
0.06
|
Continuous rainfall
|
The duration of runoff was the same as sediment yield during two continuous rainfall events. The variation of sediment yield were similar to runoff volume during two intermittent rainfall events, four runoff and sediment processes were recorded during the first rainfall event, with runoff duration of 24, 18, 7, and 14 min, respectively (Table 3). Runoff volumes were higher in the second runoff process compared to the first runoff process. The first two runoff and sediment yield accounted for 81% and 81% of total runoff and sediment yield, respectively. For the second intermittent rainfall event, runoff volumes were higher in the first runoff process compared to the second runoff process. The first two runoff and sediment yield accounted for 95% and 97% of total runoff and sediment yield, respectively.
Table 3 Runoff and sediment conditions.
Date
|
Runoff and sediment duration (minutes)
|
Runoff (L)
|
Sediment (g)
|
Runoff percentage (%)
|
Runoff percentage (%)
|
2016–07–22
|
49
|
679
|
14322
|
|
|
2016–08–13
|
32
|
816
|
34733
|
|
|
2016–07–21
|
24
|
60
|
1437
|
18
|
16
|
|
18
|
208
|
5700
|
63
|
65
|
|
7
|
36
|
801
|
11
|
9
|
|
14
|
28
|
822
|
8
|
9
|
|
63
|
332
|
8760
|
100
|
100
|
2016–07–25
|
22
|
1069
|
70732
|
88
|
88
|
|
7
|
87
|
7635
|
7
|
9
|
|
12
|
57
|
2274
|
5
|
3
|
|
41
|
1213
|
80641
|
100
|
100
|
Characteristics of runoff and sediment processes
The runoff and sediment processes with instantaneous rainfall intensity presented a pattern of multi-peaks (2 or 4 peaks) during two continuous rainfall events. For the first continuous rainfall event (Fig. 1a), the first peak value of rainfall intensity occurred at 3 min, and the first sediment yield rate at 5 min. The second peak in rainfall intensity occurred at 13 min, with runoff rate starting 2 min before (at 11 min). However, the runoff and sediment processes not have a lag in peak with rainfall intensity during the second continuous rainfall event (Fig. 1b).
For the two intermittent rainfall events, runoff processes with instantaneous rainfall intensity presented the pattern of single peak or two peaks (Figs. 2–3). The first intermittent rainfall event had four runoff processes (Fig. 2). In the first runoff process (Fig. 2a), rainfall intensity peaked at 1 min, while runoff and sediment yield rate peaked at 7 min. Runoff and sediment yield rate peaked 4 min earlier than rainfall intensity during second runoff process (Fig. 2b). For the third runoff process (Fig. 2c), rainfall intensity peaked at similar time as runoff and sediment yield rate. Rainfall intensity, runoff rate, and sediment yield rate peaked at 4, 6, and 4 min during the fourth runoff process (Fig. 2d).
Compared to the first rainfall event, three runoff processes were produced during the second intermittent rainfall event (Fig. 3). During the first runoff process (Fig. 3a), rainfall intensity peaked at 2 min and 14 min, while runoff and sediment yield rate reached the first peak, and peaked at 14 min. During the second runoff process (Fig. 3b), rainfall intensity peaked at 2 min, and runoff and sediment rate peaked at 1 min. During the third process (Fig. 3c), rainfall intensity, runoff, and sediment yield rate peaked at 4, 3, and 3 min, respectively. Sediment yield rate peaked at 11 min again, while runoff rate peaked 2 min earlier.
Rill morphological characteristics after multiple rainfall events
Rill morphological distribution on bare soil was shown in Fig. 4. Three rill types were identified as strip-shaped, V-shaped, and tree-branched distribution. Rill number increased significantly, and rill morphology gradually evolved from narrow strip to wide strip along the direction of rill head after four rill events (R1–R4). There were seven rills on the initial rill slope (R0), five of which were strip-shaped and V-shaped, and two of which were tree-branched distribution. In the first rill event (R1), the initial three (strip or V-shaped) rills merged with the two main rills to form a concentrated flow path, and a new rill with tree-branched distribution was generated, the total number of rills declining to five. With the occurrence of headward erosion, the new rills adjacent to the upper and lower branches connected to the main rills and intensely distributed in the second rill event (R2). Compared with the second rill event (R2), some rills were silted by sediment, with five rills being present in the third rill event (R3). Because of the concentrated flow and headward erosion, more complete rill development was recorded, the silted rills in the third rill event (R3) connected with the main rills again in the fourth rill event (R4).
The frequency distribution of rill width and depth in the five rill events (R0–R4) were showed in Fig. 5. Rill width was classified as six groups (0–5 cm, 5–10 cm, 10–15 cm, 15–20 cm, 20–25 cm, and > 25 cm) based on the data of rill width, rill depth was classified as 0–5 cm, 5–10 cm, and 10–15 cm. Rill width mainly ranged from 5 to 20 cm in the five rill events (R0–R4), and their proportions to the total rills were 88.89%, 85.19%, 84.13%, 85.71% and 85.72%, respectively. Rills were mainly distributed in 0–10 cm depth in four rill events (R1–R4), and the frequencies were 100.00%, 98.15%, 100.00%, 98.41% and 98.41%, respectively.
The rill parameters of bare soil in the five events (R0–R4) were shown in Table 4. After the four rill events (R1–R4), the range of rill length, width and depth were 1.45–51.57 m, 7.00–18.25 and 3.15–5.75 cm, respectively. The total and mean rill length increased 0.87 and 2.27 times compared to the initial slope (R0), as well as the mean rill width and depth increased 0.30 and 0.16 times. Mean rill length and depth showed similar trends, and rill erosion mainly reflected the variation of rill length. Rill density and width-depth ratio ranged from 0.44 to 0.83 cm and 1.73 to 2.88, respectively. The range of rill convergence and bifurcation node was 6.00–15.00 and 1.00–4.00, respectively. After the four rill events, the trend of rill density increased 0.89 times compared to the initial slope (R0), the convergence and bifurcation node also showed an increasing trend but the width-depth ratio in an opposite trend.
Table 4 The Changes of rill parameters on bare soil.
|
R0
|
R1
|
R2
|
R3
|
R4
|
ltotal
|
44.17
|
74.65
|
81.61
|
74.2
|
82.59
|
lmean
|
6.31
|
14.93
|
20.40
|
14.84
|
20.65
|
l range
|
1.45–16.70
|
1.45–44.06
|
1.45–50.56
|
1.45–45.77
|
1.45–51.57
|
Wmean
|
11.15
|
10.06
|
12.81
|
13.42
|
14.51
|
Wrange
|
7.00–16.67
|
7.50–12.00
|
11.27–14.48
|
11.50–16.25
|
11.00–18.25
|
hmean
|
4.09
|
4.53
|
4.71
|
4.31
|
4.73
|
hrange
|
3.15–5.75
|
4.00–5.15
|
4.35–5.13
|
3.25–4.75
|
4.35–5.23
|
ρ
|
0.44
|
0.75
|
0.82
|
0.74
|
0.83
|
d/h
|
2.88
|
2.23
|
1.73
|
1.90
|
1.88
|
c
|
6.00
|
10.00
|
15.00
|
13.00
|
14.00
|
b
|
1.00
|
3.00
|
4.00
|
2.00
|
3.00
|
Note: lrange, Wrange, and hrange are the range of rill length, rill width, and rill depth, respectively;Lmean, Wmean, and hmean are the mean value of rill length, rill width, and rill depth, respectively; Ltotal is the total length of rill. Rill density: ρ; Rill width-depth ratio: W/h; convergence node : c; Bifurcation node: b.
Variation of rill morphological parameters in different slope sections
The upper slope section I (0–6.67 m) was located at the upper part of bare soil, with the belt of no erosion was 0.75 m, the belt of no erosion was an area at the top of each slope that is not dissected by rills (Horton, 1945). The upper slope surface was flat with no protection from vegetation, flow was relatively uniform, and runoff erosion force was weak, while rills were densely and evenly distributed.
Compared to the slope section II (6.67–13.34 m) and III (13.34–20 m), rill density of the upper slope section I was largest (Table 5). Middle slope section II had the greatest rill number, followed by I and III. Due to the concentrated flow in the upper slope section, rills bifurcated and merged, resulting in the increased of rill number in the middle slope section II. In the lower slope section III, the rill density and number were generally lower than the other slope sections. The variation of rill width was more significant than rill depth (Fig. 6), and the value of width-depth ratio >1, indicated that side-wall collapse erosion was greater than downcutting erosion for the three slope section in the five rill event (except for the slope section III in the fourth rill event ). The variation of rill width and depth was highly obvious in middle slope section II, thus the slope section II was the most active area of rill erosion development. Because of the confluence of runoff and sediment from the upper slope section I to the lower slope section III, runoff and sediment silting reduced the rill depth to the minimum value in the lower slope section III, thus the slope section III was the sediment deposition area.
Table 5 Rill number, density and width-depth ratio of different slope sections in five rill events.
Rainfall events
|
Rill number
|
Rill density(m·m-2)
|
Rill width-depth ratio
|
I
|
II
|
III
|
I
|
II
|
III
|
I
|
II
|
III
|
R0
|
5
|
5
|
2
|
0.77
|
0.41
|
0.14
|
3.72
|
2.72
|
1.55
|
R1
|
4
|
5
|
3
|
0.90
|
0.87
|
0.48
|
3.36
|
1.97
|
1.75
|
R2
|
4
|
5
|
2
|
0.90
|
0.89
|
0.66
|
2.96
|
1.99
|
2.19
|
R3
|
5
|
5
|
3
|
0.82
|
0.87
|
0.54
|
4.11
|
2.05
|
2.49
|
R4
|
4
|
4
|
2
|
0.90
|
0.89
|
0.69
|
2.76
|
2.60
|
0.88
|